1. Field
Embodiments of the present invention relate to a lithographic apparatus, a method for reducing gas borne noise in a lithographic apparatus, and a method for manufacturing a device.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic process of a lithographic apparatus it is desirable that at least the patterning device, the projection system and the substrate stage be properly aligned with respect to each other, so that the pattern which is provided by the patterning device in the radiation beam is properly projected on a target portion of the substrate without, for example, overlay errors, imaging errors or focus errors. In particular, in scanners in which the patterning device support (reticle stage) and the substrate table (substrate stage) are movable to position a particular part of the pattern with respect to a particular part of the substrate, high accuracy positioning is desired. For these movements, positioning systems are provided which control the position of the patterning device support and substrate table with high accuracy.
With continuously increasing demands on the accuracy of imaging, for instance overlay and focus on the one hand and throughput on the other, proper alignment of patterning device support, projection system and substrate table is desirable. For increasing the throughput of the lithographic apparatus, it is desirable to increase the speed and acceleration with which the patterning device support and substrate table are moved and aligned with respect to each other and the projection system.
However, moving the patterning device support or the substrate table may result in gas flows and pressure waves which may propagate through the space in which these stages but also the projection system are present. Also, the actuation forces of the patterning device support and substrate table may cause vibrations of parts of the stage resulting in gas flows and/or pressure waves, such as acoustic signals or gas flows through the working space. These gas flows and/or pressure waves may excite the projection system, or at least parts of the projection system such as the lenses, or the frame on which the projection system is mounted. The gas flows and/or pressure waves may also excite other parts of the lithographic apparatus relevant for the alignment of the patterning device support, projection system and substrate table such as a sensor or sensor target object of a stage position measurement system. The excitation of the projection system, or the other parts, may cause imaging errors such as errors in overlay, fading and/or focus.
Generally, it has been found that there are three sources for excitation of the projection system by movements of the patterning device support. One source type is gas borne noise, i.e. noise borne by the gas present between the patterning device support and the projection system, for instance air. This gas borne noise is typically the result of the acceleration of the patterning device support, whereby the gas column before the patterning device support in the direction of movement is pressed away by the moving patterning device support, while gas is sucked into the space created by the moving patterning device support. This gas borne noise may typically be low frequency noise, for instance lower than 150 Hz, and may result in overlay errors.
Another source type is structure borne noise. The movement of the patterning device support and the forces desired for that movement may result in excitation of a number of parts of the patterning device support or the support structure of the patterning device support. This excitation may be transferred to the projection system via the structural transmission path or via the acoustics transmission path of the lithographic apparatus, for instance a frame or combination of frames which supports both the patterning device support and the projection system. This structure borne noise may typically be of a higher frequency range, for instance above 150 Hz and may have a negative effect on overlay and/or fading.
Furthermore, the excitation of the parts of the patterning device support or the support structure of the patterning device support may also result in vibrations of these parts. These vibrating parts may also cause pressure waves that excite the projection system or other elements. This results in relative high frequency vibrations of the projection system or other elements, causing a negative effect on overlay and/or fading.
A third source type is vortex shedding or turbulences, caused by sharp edges of the moveable object. This may generate noise typically of a higher frequency range, for instance above 500 Hz.